This may count as simply a typo, so sorry if it does:
Does ò73f(x)dx or ò73(x + 10)dx (bottom of page 53) mean integral as shown in the following diagram? I found it in the live book version and my downloaded PDF.
CR

As a side remark, it’s a bit of a shame that you have to write f.apply(a) instead of just f(a)
as in mathematics, but Java just can’t get away from the view that everything is an object

(page 55).

In some respects, Java is getting to grips with the idea that ints and other primitives might be objects: was it Project Valhalla?
On the other hand, maybe in three years it will be possible to write f(x) as you suggested.